WASHINGTON — Despite a big-bucks lobby blitz that includes coffee chats, phone calls and letters, 85 conservative House freshmen, including New York’s five Tea Party-backed Republicans, aren’t budging in their opposition to raising the federal debt limit.

“It simply is not an option,” said Rep. Mike Grimm (R-SI), warning that crushing debt is just as big an economic threat as a federal credit default from not raising the $14 trillion limit.

“I have to hold my ground because otherwise, I really believe . . . our economy will be destroyed,” Grimm said. “Everyone — every economist in the world — everyone recognizes that our debt is unsustainable.”

Grimm isn’t the only GOP greenhorn who shows no sign of caving to the debt-limit push by Wall Street, big business and the Treasury Department.

Upstate Republican Rep. Ann Marie Buerkle said she won’t OK more debt without “deep cuts,” adding that is what voters demand.

“Every time we raise that debt ceiling, we’re spending money we don’t have,” Buerkle told The Post. “The American people get it. Now the Senate and the president have to get it.”

Grimm, Buerkle and their freshman colleagues vow not to raise the debt ceiling unless President Obama and the Democratic-led Senate agree to severe spending cuts and fundamental budget reforms.

Their stance is in line with that of Republican leaders who won the House last year, thanks to the plucky Tea Party freshmen.

Congress will have to vote as soon as next month on whether to increase the debt limit when the government hits the $14.3 trillion ceiling.

Echoing stark warnings from Obama and his Democratic allies, the lobbyists admonish freshmen that voting “no” would be catastrophic.

It would cut off the government’s credit line. The United States would be unable to pay its foreign creditors, such as China. The US credit rating would plummet — causing the government to pay higher interest on the debt.

The doomsday scenario so far isn’t scaring the new Republican House members.

They are sticking to the Tea Party principles that got them elected: Control spending, cut deficits and debt, keep taxes low, shrink government.

“The House majority takes the firm stance that we must act now to stop adding to the debt,” said Rep. Nan Hayworth (R-Goshen).

“Clearly, it is going to take time and courage and dedication to reach a point of a balanced federal budget. But we have to act in that way.”